1. Field of the Invention
The present invention relates to a method and apparatus for evaluation of schlieren in optical materials by means of shadow methods.
2. Description of the Related Art
Schlieren, according to conventional speech usage, are bounded regions in optical materials, which act optically because of local changes in refractive index and are mostly visible in an image or picture in the form of filaments, strips and bands. Nearly all optical materials do not have an index of refraction that is exactly constant throughout, but instead it varies within a certain range. Schlieren are defined as optical inhomogeneities of small structural width and a high refractive index gradient.
A variety of different methods are known for detection of schlieren in optical materials. Practically all these methods are based on detecting changes of the optical wave front after passing through the sample to be tested and analyzed. Interferometry, Töpler methods and shadow methods are the most widely used methods.
Changes in the wave front are detected directly by means of interferometry and Töpler methods; however surface imperfections and faults on the test sample, such as surfaces that are not completely planar, effect the measurement directly. These measurements involve great effort and expense because of the high requirements for surface uniformity of the test sample.
Usually commercially obtained interferometers are not ready for schlieren measurements; i.e. if schlieren must be measured with an interferometer, it must be constructed for that purpose. Thus it is important that spatial resolution of the interferometer is sufficient so that fine schlieren structures can be detected, whose size is in the micron range.
To detect schlieren it is preferably to use shadow methods, because they are comparatively sensitive and can be performed when the surface quality of the samples is similar to that of typical optical materials.
In shadow methods the optical material is either between a light source and the eye of an observer and the shadow casting schlierens are established by moving and tilting the sample (MIL-G-174A and similar standards), or the sample is irradiated with light and the schlieren contained in the sample are projected as shadows (DDR Professional Standard TGL 21790, similar ISO standard is widely distributed).
Also DIN 3140, Part 3, concerns schlieren, however in practice has only a limited significance.
Since schlieren are spatial formations, the methods described in Standard TGL 21790 and also in DIN 3140 attempt to characterize the extent of the schlieren by an effective schlieren surface or area. Known procedures have the disadvantage that they depend strongly on the subjective evaluation of the observer. In Standard TGL 21790 definite schlieren comparisons are derived from measurement of visibility threshold. However the comparison of the schlieren image with a test pattern occurs in the known methods only by eye and thus depends on the subjective observation powers of the observer to some extent.
In the conventional procedure the shadow image of the respective test sample is copied onto a piece of paper, the width and the intensity of the individual shadow lines is subjectively evaluated and the evaluation or analysis is registered on the sketch.
Furthermore the above-described standard is exclusively related to glass and glass-like substances. However increasingly crystalline materials are used for optical components, especially for wavelengths, which are outside of the visible range, which means substantially below about 400 nm and above about 800 nm. Thus, for example, there is an increasing demand for monocrystalline materials made of alkali and alkaline earth fluorides (CaF2, BaF2, SrF2, among others) for UV applications, such as UV lithography or lenses and widows for irradiation and imaging apparatuses. Crystals provide the basis for many optical elements in the IR spectral range.
Glasses and crystals differ by their respective disorderly and orderly structures. Schlieren in crystals can have entirely different causes than schlieren in glass. The activity of schlieren in crystals depends, among other things, very strongly on the position and orientation of the inhomogeneities producing the schlieren (e.g. grain boundaries). In crystals schlieren can be produced by band-shaped structures of greater width, but of only reduced thickness (e.g. displacements, small angle grain boundaries). The methods developed for glasses are not suitable to analyze substrates with schlieren of this type permeating them with the required accuracy.
Also the test or comparison schlieren samples used for glasses are not usable for crystals because of the completely different mechanisms for producing schlieren in crystals. A coating in the known comparative schlieren test plate developed for testing optical glass simulates phase discontinuities of different thicknesses and widths. However one such two-dimensional schlieren test plate is not suitable for representing the action of schlieren in crystals.
Especially during testing for the present invention the phase deviation or shift has the opposite sign in single crystalline materials from that in glasses. The schlieren test and comparison plate known up to now however usually is made from an insensitive material, for example quartz or quartz glass. Different structures are provided in this plate, e.g. by means of a thin coating and a photo mask. Usually cavities or openings of e.g. 0.2 mm width and 1 cm length are provided in a 10 nm coating, whereby a phase shift is produced when a wave front passes through. In principle, it is also possible to produce a phase shift by application of raised regions instead of cavities or openings, whereby the sign of the shift changes. The subjective intensity and width of the lines in the shadow image evaluated with the help of the comparison or test plate however has no reliable significance for test samples made from crystalline material, since a change of the light wave front is caused by a change in the index of refraction. It has been found in the scope of the invention that the results obtained by comparison with known schlieren test plates with the shadow methods for crystals are mostly unusable and unreliable. In practice they produce no relevant conclusions regarding the quality of the crystalline optical elements.